The Design and Implementation of the FreeBSD Operating System, Second Edition
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FreeBSD/Linux Kernel Cross Reference
sys/netinet/tcp_syncache.c

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    1 /*-
    2  * Copyright (c) 2001 McAfee, Inc.
    3  * Copyright (c) 2006,2013 Andre Oppermann, Internet Business Solutions AG
    4  * All rights reserved.
    5  *
    6  * This software was developed for the FreeBSD Project by Jonathan Lemon
    7  * and McAfee Research, the Security Research Division of McAfee, Inc. under
    8  * DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
    9  * DARPA CHATS research program. [2001 McAfee, Inc.]
   10  *
   11  * Redistribution and use in source and binary forms, with or without
   12  * modification, are permitted provided that the following conditions
   13  * are met:
   14  * 1. Redistributions of source code must retain the above copyright
   15  *    notice, this list of conditions and the following disclaimer.
   16  * 2. Redistributions in binary form must reproduce the above copyright
   17  *    notice, this list of conditions and the following disclaimer in the
   18  *    documentation and/or other materials provided with the distribution.
   19  *
   20  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
   21  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
   22  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
   23  * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
   24  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
   25  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
   26  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
   27  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   28  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
   29  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
   30  * SUCH DAMAGE.
   31  */
   32 
   33 #include <sys/cdefs.h>
   34 __FBSDID("$FreeBSD: releng/10.0/sys/netinet/tcp_syncache.c 254889 2013-08-25 21:54:41Z markj $");
   35 
   36 #include "opt_inet.h"
   37 #include "opt_inet6.h"
   38 #include "opt_ipsec.h"
   39 #include "opt_pcbgroup.h"
   40 
   41 #include <sys/param.h>
   42 #include <sys/systm.h>
   43 #include <sys/kernel.h>
   44 #include <sys/sysctl.h>
   45 #include <sys/limits.h>
   46 #include <sys/lock.h>
   47 #include <sys/mutex.h>
   48 #include <sys/malloc.h>
   49 #include <sys/mbuf.h>
   50 #include <sys/proc.h>           /* for proc0 declaration */
   51 #include <sys/random.h>
   52 #include <sys/socket.h>
   53 #include <sys/socketvar.h>
   54 #include <sys/syslog.h>
   55 #include <sys/ucred.h>
   56 
   57 #include <sys/md5.h>
   58 #include <crypto/siphash/siphash.h>
   59 
   60 #include <vm/uma.h>
   61 
   62 #include <net/if.h>
   63 #include <net/route.h>
   64 #include <net/vnet.h>
   65 
   66 #include <netinet/in.h>
   67 #include <netinet/in_systm.h>
   68 #include <netinet/ip.h>
   69 #include <netinet/in_var.h>
   70 #include <netinet/in_pcb.h>
   71 #include <netinet/ip_var.h>
   72 #include <netinet/ip_options.h>
   73 #ifdef INET6
   74 #include <netinet/ip6.h>
   75 #include <netinet/icmp6.h>
   76 #include <netinet6/nd6.h>
   77 #include <netinet6/ip6_var.h>
   78 #include <netinet6/in6_pcb.h>
   79 #endif
   80 #include <netinet/tcp.h>
   81 #include <netinet/tcp_fsm.h>
   82 #include <netinet/tcp_seq.h>
   83 #include <netinet/tcp_timer.h>
   84 #include <netinet/tcp_var.h>
   85 #include <netinet/tcp_syncache.h>
   86 #ifdef INET6
   87 #include <netinet6/tcp6_var.h>
   88 #endif
   89 #ifdef TCP_OFFLOAD
   90 #include <netinet/toecore.h>
   91 #endif
   92 
   93 #ifdef IPSEC
   94 #include <netipsec/ipsec.h>
   95 #ifdef INET6
   96 #include <netipsec/ipsec6.h>
   97 #endif
   98 #include <netipsec/key.h>
   99 #endif /*IPSEC*/
  100 
  101 #include <machine/in_cksum.h>
  102 
  103 #include <security/mac/mac_framework.h>
  104 
  105 static VNET_DEFINE(int, tcp_syncookies) = 1;
  106 #define V_tcp_syncookies                VNET(tcp_syncookies)
  107 SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, syncookies, CTLFLAG_RW,
  108     &VNET_NAME(tcp_syncookies), 0,
  109     "Use TCP SYN cookies if the syncache overflows");
  110 
  111 static VNET_DEFINE(int, tcp_syncookiesonly) = 0;
  112 #define V_tcp_syncookiesonly            VNET(tcp_syncookiesonly)
  113 SYSCTL_VNET_INT(_net_inet_tcp, OID_AUTO, syncookies_only, CTLFLAG_RW,
  114     &VNET_NAME(tcp_syncookiesonly), 0,
  115     "Use only TCP SYN cookies");
  116 
  117 #ifdef TCP_OFFLOAD
  118 #define ADDED_BY_TOE(sc) ((sc)->sc_tod != NULL)
  119 #endif
  120 
  121 static void      syncache_drop(struct syncache *, struct syncache_head *);
  122 static void      syncache_free(struct syncache *);
  123 static void      syncache_insert(struct syncache *, struct syncache_head *);
  124 struct syncache *syncache_lookup(struct in_conninfo *, struct syncache_head **);
  125 static int       syncache_respond(struct syncache *);
  126 static struct    socket *syncache_socket(struct syncache *, struct socket *,
  127                     struct mbuf *m);
  128 static int       syncache_sysctl_count(SYSCTL_HANDLER_ARGS);
  129 static void      syncache_timeout(struct syncache *sc, struct syncache_head *sch,
  130                     int docallout);
  131 static void      syncache_timer(void *);
  132 
  133 static uint32_t  syncookie_mac(struct in_conninfo *, tcp_seq, uint8_t,
  134                     uint8_t *, uintptr_t);
  135 static tcp_seq   syncookie_generate(struct syncache_head *, struct syncache *);
  136 static struct syncache
  137                 *syncookie_lookup(struct in_conninfo *, struct syncache_head *,
  138                     struct syncache *, struct tcphdr *, struct tcpopt *,
  139                     struct socket *);
  140 static void      syncookie_reseed(void *);
  141 #ifdef INVARIANTS
  142 static int       syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
  143                     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
  144                     struct socket *lso);
  145 #endif
  146 
  147 /*
  148  * Transmit the SYN,ACK fewer times than TCP_MAXRXTSHIFT specifies.
  149  * 3 retransmits corresponds to a timeout of 3 * (1 + 2 + 4 + 8) == 45 seconds,
  150  * the odds are that the user has given up attempting to connect by then.
  151  */
  152 #define SYNCACHE_MAXREXMTS              3
  153 
  154 /* Arbitrary values */
  155 #define TCP_SYNCACHE_HASHSIZE           512
  156 #define TCP_SYNCACHE_BUCKETLIMIT        30
  157 
  158 static VNET_DEFINE(struct tcp_syncache, tcp_syncache);
  159 #define V_tcp_syncache                  VNET(tcp_syncache)
  160 
  161 static SYSCTL_NODE(_net_inet_tcp, OID_AUTO, syncache, CTLFLAG_RW, 0,
  162     "TCP SYN cache");
  163 
  164 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, bucketlimit, CTLFLAG_RDTUN,
  165     &VNET_NAME(tcp_syncache.bucket_limit), 0,
  166     "Per-bucket hash limit for syncache");
  167 
  168 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, cachelimit, CTLFLAG_RDTUN,
  169     &VNET_NAME(tcp_syncache.cache_limit), 0,
  170     "Overall entry limit for syncache");
  171 
  172 SYSCTL_VNET_PROC(_net_inet_tcp_syncache, OID_AUTO, count, (CTLTYPE_UINT|CTLFLAG_RD),
  173     NULL, 0, &syncache_sysctl_count, "IU",
  174     "Current number of entries in syncache");
  175 
  176 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, hashsize, CTLFLAG_RDTUN,
  177     &VNET_NAME(tcp_syncache.hashsize), 0,
  178     "Size of TCP syncache hashtable");
  179 
  180 SYSCTL_VNET_UINT(_net_inet_tcp_syncache, OID_AUTO, rexmtlimit, CTLFLAG_RW,
  181     &VNET_NAME(tcp_syncache.rexmt_limit), 0,
  182     "Limit on SYN/ACK retransmissions");
  183 
  184 VNET_DEFINE(int, tcp_sc_rst_sock_fail) = 1;
  185 SYSCTL_VNET_INT(_net_inet_tcp_syncache, OID_AUTO, rst_on_sock_fail,
  186     CTLFLAG_RW, &VNET_NAME(tcp_sc_rst_sock_fail), 0,
  187     "Send reset on socket allocation failure");
  188 
  189 static MALLOC_DEFINE(M_SYNCACHE, "syncache", "TCP syncache");
  190 
  191 #define SYNCACHE_HASH(inc, mask)                                        \
  192         ((V_tcp_syncache.hash_secret ^                                  \
  193           (inc)->inc_faddr.s_addr ^                                     \
  194           ((inc)->inc_faddr.s_addr >> 16) ^                             \
  195           (inc)->inc_fport ^ (inc)->inc_lport) & mask)
  196 
  197 #define SYNCACHE_HASH6(inc, mask)                                       \
  198         ((V_tcp_syncache.hash_secret ^                                  \
  199           (inc)->inc6_faddr.s6_addr32[0] ^                              \
  200           (inc)->inc6_faddr.s6_addr32[3] ^                              \
  201           (inc)->inc_fport ^ (inc)->inc_lport) & mask)
  202 
  203 #define ENDPTS_EQ(a, b) (                                               \
  204         (a)->ie_fport == (b)->ie_fport &&                               \
  205         (a)->ie_lport == (b)->ie_lport &&                               \
  206         (a)->ie_faddr.s_addr == (b)->ie_faddr.s_addr &&                 \
  207         (a)->ie_laddr.s_addr == (b)->ie_laddr.s_addr                    \
  208 )
  209 
  210 #define ENDPTS6_EQ(a, b) (memcmp(a, b, sizeof(*a)) == 0)
  211 
  212 #define SCH_LOCK(sch)           mtx_lock(&(sch)->sch_mtx)
  213 #define SCH_UNLOCK(sch)         mtx_unlock(&(sch)->sch_mtx)
  214 #define SCH_LOCK_ASSERT(sch)    mtx_assert(&(sch)->sch_mtx, MA_OWNED)
  215 
  216 /*
  217  * Requires the syncache entry to be already removed from the bucket list.
  218  */
  219 static void
  220 syncache_free(struct syncache *sc)
  221 {
  222 
  223         if (sc->sc_ipopts)
  224                 (void) m_free(sc->sc_ipopts);
  225         if (sc->sc_cred)
  226                 crfree(sc->sc_cred);
  227 #ifdef MAC
  228         mac_syncache_destroy(&sc->sc_label);
  229 #endif
  230 
  231         uma_zfree(V_tcp_syncache.zone, sc);
  232 }
  233 
  234 void
  235 syncache_init(void)
  236 {
  237         int i;
  238 
  239         V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
  240         V_tcp_syncache.bucket_limit = TCP_SYNCACHE_BUCKETLIMIT;
  241         V_tcp_syncache.rexmt_limit = SYNCACHE_MAXREXMTS;
  242         V_tcp_syncache.hash_secret = arc4random();
  243 
  244         TUNABLE_INT_FETCH("net.inet.tcp.syncache.hashsize",
  245             &V_tcp_syncache.hashsize);
  246         TUNABLE_INT_FETCH("net.inet.tcp.syncache.bucketlimit",
  247             &V_tcp_syncache.bucket_limit);
  248         if (!powerof2(V_tcp_syncache.hashsize) ||
  249             V_tcp_syncache.hashsize == 0) {
  250                 printf("WARNING: syncache hash size is not a power of 2.\n");
  251                 V_tcp_syncache.hashsize = TCP_SYNCACHE_HASHSIZE;
  252         }
  253         V_tcp_syncache.hashmask = V_tcp_syncache.hashsize - 1;
  254 
  255         /* Set limits. */
  256         V_tcp_syncache.cache_limit =
  257             V_tcp_syncache.hashsize * V_tcp_syncache.bucket_limit;
  258         TUNABLE_INT_FETCH("net.inet.tcp.syncache.cachelimit",
  259             &V_tcp_syncache.cache_limit);
  260 
  261         /* Allocate the hash table. */
  262         V_tcp_syncache.hashbase = malloc(V_tcp_syncache.hashsize *
  263             sizeof(struct syncache_head), M_SYNCACHE, M_WAITOK | M_ZERO);
  264 
  265 #ifdef VIMAGE
  266         V_tcp_syncache.vnet = curvnet;
  267 #endif
  268 
  269         /* Initialize the hash buckets. */
  270         for (i = 0; i < V_tcp_syncache.hashsize; i++) {
  271                 TAILQ_INIT(&V_tcp_syncache.hashbase[i].sch_bucket);
  272                 mtx_init(&V_tcp_syncache.hashbase[i].sch_mtx, "tcp_sc_head",
  273                          NULL, MTX_DEF);
  274                 callout_init_mtx(&V_tcp_syncache.hashbase[i].sch_timer,
  275                          &V_tcp_syncache.hashbase[i].sch_mtx, 0);
  276                 V_tcp_syncache.hashbase[i].sch_length = 0;
  277                 V_tcp_syncache.hashbase[i].sch_sc = &V_tcp_syncache;
  278         }
  279 
  280         /* Create the syncache entry zone. */
  281         V_tcp_syncache.zone = uma_zcreate("syncache", sizeof(struct syncache),
  282             NULL, NULL, NULL, NULL, UMA_ALIGN_PTR, 0);
  283         V_tcp_syncache.cache_limit = uma_zone_set_max(V_tcp_syncache.zone,
  284             V_tcp_syncache.cache_limit);
  285 
  286         /* Start the SYN cookie reseeder callout. */
  287         callout_init(&V_tcp_syncache.secret.reseed, 1);
  288         arc4rand(V_tcp_syncache.secret.key[0], SYNCOOKIE_SECRET_SIZE, 0);
  289         arc4rand(V_tcp_syncache.secret.key[1], SYNCOOKIE_SECRET_SIZE, 0);
  290         callout_reset(&V_tcp_syncache.secret.reseed, SYNCOOKIE_LIFETIME * hz,
  291             syncookie_reseed, &V_tcp_syncache);
  292 }
  293 
  294 #ifdef VIMAGE
  295 void
  296 syncache_destroy(void)
  297 {
  298         struct syncache_head *sch;
  299         struct syncache *sc, *nsc;
  300         int i;
  301 
  302         /* Cleanup hash buckets: stop timers, free entries, destroy locks. */
  303         for (i = 0; i < V_tcp_syncache.hashsize; i++) {
  304 
  305                 sch = &V_tcp_syncache.hashbase[i];
  306                 callout_drain(&sch->sch_timer);
  307 
  308                 SCH_LOCK(sch);
  309                 TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc)
  310                         syncache_drop(sc, sch);
  311                 SCH_UNLOCK(sch);
  312                 KASSERT(TAILQ_EMPTY(&sch->sch_bucket),
  313                     ("%s: sch->sch_bucket not empty", __func__));
  314                 KASSERT(sch->sch_length == 0, ("%s: sch->sch_length %d not 0",
  315                     __func__, sch->sch_length));
  316                 mtx_destroy(&sch->sch_mtx);
  317         }
  318 
  319         KASSERT(uma_zone_get_cur(V_tcp_syncache.zone) == 0,
  320             ("%s: cache_count not 0", __func__));
  321 
  322         /* Free the allocated global resources. */
  323         uma_zdestroy(V_tcp_syncache.zone);
  324         free(V_tcp_syncache.hashbase, M_SYNCACHE);
  325 
  326         callout_drain(&V_tcp_syncache.secret.reseed);
  327 }
  328 #endif
  329 
  330 static int
  331 syncache_sysctl_count(SYSCTL_HANDLER_ARGS)
  332 {
  333         int count;
  334 
  335         count = uma_zone_get_cur(V_tcp_syncache.zone);
  336         return (sysctl_handle_int(oidp, &count, 0, req));
  337 }
  338 
  339 /*
  340  * Inserts a syncache entry into the specified bucket row.
  341  * Locks and unlocks the syncache_head autonomously.
  342  */
  343 static void
  344 syncache_insert(struct syncache *sc, struct syncache_head *sch)
  345 {
  346         struct syncache *sc2;
  347 
  348         SCH_LOCK(sch);
  349 
  350         /*
  351          * Make sure that we don't overflow the per-bucket limit.
  352          * If the bucket is full, toss the oldest element.
  353          */
  354         if (sch->sch_length >= V_tcp_syncache.bucket_limit) {
  355                 KASSERT(!TAILQ_EMPTY(&sch->sch_bucket),
  356                         ("sch->sch_length incorrect"));
  357                 sc2 = TAILQ_LAST(&sch->sch_bucket, sch_head);
  358                 syncache_drop(sc2, sch);
  359                 TCPSTAT_INC(tcps_sc_bucketoverflow);
  360         }
  361 
  362         /* Put it into the bucket. */
  363         TAILQ_INSERT_HEAD(&sch->sch_bucket, sc, sc_hash);
  364         sch->sch_length++;
  365 
  366 #ifdef TCP_OFFLOAD
  367         if (ADDED_BY_TOE(sc)) {
  368                 struct toedev *tod = sc->sc_tod;
  369 
  370                 tod->tod_syncache_added(tod, sc->sc_todctx);
  371         }
  372 #endif
  373 
  374         /* Reinitialize the bucket row's timer. */
  375         if (sch->sch_length == 1)
  376                 sch->sch_nextc = ticks + INT_MAX;
  377         syncache_timeout(sc, sch, 1);
  378 
  379         SCH_UNLOCK(sch);
  380 
  381         TCPSTAT_INC(tcps_sc_added);
  382 }
  383 
  384 /*
  385  * Remove and free entry from syncache bucket row.
  386  * Expects locked syncache head.
  387  */
  388 static void
  389 syncache_drop(struct syncache *sc, struct syncache_head *sch)
  390 {
  391 
  392         SCH_LOCK_ASSERT(sch);
  393 
  394         TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
  395         sch->sch_length--;
  396 
  397 #ifdef TCP_OFFLOAD
  398         if (ADDED_BY_TOE(sc)) {
  399                 struct toedev *tod = sc->sc_tod;
  400 
  401                 tod->tod_syncache_removed(tod, sc->sc_todctx);
  402         }
  403 #endif
  404 
  405         syncache_free(sc);
  406 }
  407 
  408 /*
  409  * Engage/reengage time on bucket row.
  410  */
  411 static void
  412 syncache_timeout(struct syncache *sc, struct syncache_head *sch, int docallout)
  413 {
  414         sc->sc_rxttime = ticks +
  415                 TCPTV_RTOBASE * (tcp_syn_backoff[sc->sc_rxmits]);
  416         sc->sc_rxmits++;
  417         if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc)) {
  418                 sch->sch_nextc = sc->sc_rxttime;
  419                 if (docallout)
  420                         callout_reset(&sch->sch_timer, sch->sch_nextc - ticks,
  421                             syncache_timer, (void *)sch);
  422         }
  423 }
  424 
  425 /*
  426  * Walk the timer queues, looking for SYN,ACKs that need to be retransmitted.
  427  * If we have retransmitted an entry the maximum number of times, expire it.
  428  * One separate timer for each bucket row.
  429  */
  430 static void
  431 syncache_timer(void *xsch)
  432 {
  433         struct syncache_head *sch = (struct syncache_head *)xsch;
  434         struct syncache *sc, *nsc;
  435         int tick = ticks;
  436         char *s;
  437 
  438         CURVNET_SET(sch->sch_sc->vnet);
  439 
  440         /* NB: syncache_head has already been locked by the callout. */
  441         SCH_LOCK_ASSERT(sch);
  442 
  443         /*
  444          * In the following cycle we may remove some entries and/or
  445          * advance some timeouts, so re-initialize the bucket timer.
  446          */
  447         sch->sch_nextc = tick + INT_MAX;
  448 
  449         TAILQ_FOREACH_SAFE(sc, &sch->sch_bucket, sc_hash, nsc) {
  450                 /*
  451                  * We do not check if the listen socket still exists
  452                  * and accept the case where the listen socket may be
  453                  * gone by the time we resend the SYN/ACK.  We do
  454                  * not expect this to happens often. If it does,
  455                  * then the RST will be sent by the time the remote
  456                  * host does the SYN/ACK->ACK.
  457                  */
  458                 if (TSTMP_GT(sc->sc_rxttime, tick)) {
  459                         if (TSTMP_LT(sc->sc_rxttime, sch->sch_nextc))
  460                                 sch->sch_nextc = sc->sc_rxttime;
  461                         continue;
  462                 }
  463                 if (sc->sc_rxmits > V_tcp_syncache.rexmt_limit) {
  464                         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  465                                 log(LOG_DEBUG, "%s; %s: Retransmits exhausted, "
  466                                     "giving up and removing syncache entry\n",
  467                                     s, __func__);
  468                                 free(s, M_TCPLOG);
  469                         }
  470                         syncache_drop(sc, sch);
  471                         TCPSTAT_INC(tcps_sc_stale);
  472                         continue;
  473                 }
  474                 if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  475                         log(LOG_DEBUG, "%s; %s: Response timeout, "
  476                             "retransmitting (%u) SYN|ACK\n",
  477                             s, __func__, sc->sc_rxmits);
  478                         free(s, M_TCPLOG);
  479                 }
  480 
  481                 (void) syncache_respond(sc);
  482                 TCPSTAT_INC(tcps_sc_retransmitted);
  483                 syncache_timeout(sc, sch, 0);
  484         }
  485         if (!TAILQ_EMPTY(&(sch)->sch_bucket))
  486                 callout_reset(&(sch)->sch_timer, (sch)->sch_nextc - tick,
  487                         syncache_timer, (void *)(sch));
  488         CURVNET_RESTORE();
  489 }
  490 
  491 /*
  492  * Find an entry in the syncache.
  493  * Returns always with locked syncache_head plus a matching entry or NULL.
  494  */
  495 struct syncache *
  496 syncache_lookup(struct in_conninfo *inc, struct syncache_head **schp)
  497 {
  498         struct syncache *sc;
  499         struct syncache_head *sch;
  500 
  501 #ifdef INET6
  502         if (inc->inc_flags & INC_ISIPV6) {
  503                 sch = &V_tcp_syncache.hashbase[
  504                     SYNCACHE_HASH6(inc, V_tcp_syncache.hashmask)];
  505                 *schp = sch;
  506 
  507                 SCH_LOCK(sch);
  508 
  509                 /* Circle through bucket row to find matching entry. */
  510                 TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
  511                         if (ENDPTS6_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
  512                                 return (sc);
  513                 }
  514         } else
  515 #endif
  516         {
  517                 sch = &V_tcp_syncache.hashbase[
  518                     SYNCACHE_HASH(inc, V_tcp_syncache.hashmask)];
  519                 *schp = sch;
  520 
  521                 SCH_LOCK(sch);
  522 
  523                 /* Circle through bucket row to find matching entry. */
  524                 TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
  525 #ifdef INET6
  526                         if (sc->sc_inc.inc_flags & INC_ISIPV6)
  527                                 continue;
  528 #endif
  529                         if (ENDPTS_EQ(&inc->inc_ie, &sc->sc_inc.inc_ie))
  530                                 return (sc);
  531                 }
  532         }
  533         SCH_LOCK_ASSERT(*schp);
  534         return (NULL);                  /* always returns with locked sch */
  535 }
  536 
  537 /*
  538  * This function is called when we get a RST for a
  539  * non-existent connection, so that we can see if the
  540  * connection is in the syn cache.  If it is, zap it.
  541  */
  542 void
  543 syncache_chkrst(struct in_conninfo *inc, struct tcphdr *th)
  544 {
  545         struct syncache *sc;
  546         struct syncache_head *sch;
  547         char *s = NULL;
  548 
  549         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  550         SCH_LOCK_ASSERT(sch);
  551 
  552         /*
  553          * Any RST to our SYN|ACK must not carry ACK, SYN or FIN flags.
  554          * See RFC 793 page 65, section SEGMENT ARRIVES.
  555          */
  556         if (th->th_flags & (TH_ACK|TH_SYN|TH_FIN)) {
  557                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  558                         log(LOG_DEBUG, "%s; %s: Spurious RST with ACK, SYN or "
  559                             "FIN flag set, segment ignored\n", s, __func__);
  560                 TCPSTAT_INC(tcps_badrst);
  561                 goto done;
  562         }
  563 
  564         /*
  565          * No corresponding connection was found in syncache.
  566          * If syncookies are enabled and possibly exclusively
  567          * used, or we are under memory pressure, a valid RST
  568          * may not find a syncache entry.  In that case we're
  569          * done and no SYN|ACK retransmissions will happen.
  570          * Otherwise the RST was misdirected or spoofed.
  571          */
  572         if (sc == NULL) {
  573                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  574                         log(LOG_DEBUG, "%s; %s: Spurious RST without matching "
  575                             "syncache entry (possibly syncookie only), "
  576                             "segment ignored\n", s, __func__);
  577                 TCPSTAT_INC(tcps_badrst);
  578                 goto done;
  579         }
  580 
  581         /*
  582          * If the RST bit is set, check the sequence number to see
  583          * if this is a valid reset segment.
  584          * RFC 793 page 37:
  585          *   In all states except SYN-SENT, all reset (RST) segments
  586          *   are validated by checking their SEQ-fields.  A reset is
  587          *   valid if its sequence number is in the window.
  588          *
  589          *   The sequence number in the reset segment is normally an
  590          *   echo of our outgoing acknowlegement numbers, but some hosts
  591          *   send a reset with the sequence number at the rightmost edge
  592          *   of our receive window, and we have to handle this case.
  593          */
  594         if (SEQ_GEQ(th->th_seq, sc->sc_irs) &&
  595             SEQ_LEQ(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
  596                 syncache_drop(sc, sch);
  597                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  598                         log(LOG_DEBUG, "%s; %s: Our SYN|ACK was rejected, "
  599                             "connection attempt aborted by remote endpoint\n",
  600                             s, __func__);
  601                 TCPSTAT_INC(tcps_sc_reset);
  602         } else {
  603                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  604                         log(LOG_DEBUG, "%s; %s: RST with invalid SEQ %u != "
  605                             "IRS %u (+WND %u), segment ignored\n",
  606                             s, __func__, th->th_seq, sc->sc_irs, sc->sc_wnd);
  607                 TCPSTAT_INC(tcps_badrst);
  608         }
  609 
  610 done:
  611         if (s != NULL)
  612                 free(s, M_TCPLOG);
  613         SCH_UNLOCK(sch);
  614 }
  615 
  616 void
  617 syncache_badack(struct in_conninfo *inc)
  618 {
  619         struct syncache *sc;
  620         struct syncache_head *sch;
  621 
  622         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  623         SCH_LOCK_ASSERT(sch);
  624         if (sc != NULL) {
  625                 syncache_drop(sc, sch);
  626                 TCPSTAT_INC(tcps_sc_badack);
  627         }
  628         SCH_UNLOCK(sch);
  629 }
  630 
  631 void
  632 syncache_unreach(struct in_conninfo *inc, struct tcphdr *th)
  633 {
  634         struct syncache *sc;
  635         struct syncache_head *sch;
  636 
  637         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  638         SCH_LOCK_ASSERT(sch);
  639         if (sc == NULL)
  640                 goto done;
  641 
  642         /* If the sequence number != sc_iss, then it's a bogus ICMP msg */
  643         if (ntohl(th->th_seq) != sc->sc_iss)
  644                 goto done;
  645 
  646         /*
  647          * If we've rertransmitted 3 times and this is our second error,
  648          * we remove the entry.  Otherwise, we allow it to continue on.
  649          * This prevents us from incorrectly nuking an entry during a
  650          * spurious network outage.
  651          *
  652          * See tcp_notify().
  653          */
  654         if ((sc->sc_flags & SCF_UNREACH) == 0 || sc->sc_rxmits < 3 + 1) {
  655                 sc->sc_flags |= SCF_UNREACH;
  656                 goto done;
  657         }
  658         syncache_drop(sc, sch);
  659         TCPSTAT_INC(tcps_sc_unreach);
  660 done:
  661         SCH_UNLOCK(sch);
  662 }
  663 
  664 /*
  665  * Build a new TCP socket structure from a syncache entry.
  666  */
  667 static struct socket *
  668 syncache_socket(struct syncache *sc, struct socket *lso, struct mbuf *m)
  669 {
  670         struct inpcb *inp = NULL;
  671         struct socket *so;
  672         struct tcpcb *tp;
  673         int error;
  674         char *s;
  675 
  676         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
  677 
  678         /*
  679          * Ok, create the full blown connection, and set things up
  680          * as they would have been set up if we had created the
  681          * connection when the SYN arrived.  If we can't create
  682          * the connection, abort it.
  683          */
  684         so = sonewconn(lso, SS_ISCONNECTED);
  685         if (so == NULL) {
  686                 /*
  687                  * Drop the connection; we will either send a RST or
  688                  * have the peer retransmit its SYN again after its
  689                  * RTO and try again.
  690                  */
  691                 TCPSTAT_INC(tcps_listendrop);
  692                 if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  693                         log(LOG_DEBUG, "%s; %s: Socket create failed "
  694                             "due to limits or memory shortage\n",
  695                             s, __func__);
  696                         free(s, M_TCPLOG);
  697                 }
  698                 goto abort2;
  699         }
  700 #ifdef MAC
  701         mac_socketpeer_set_from_mbuf(m, so);
  702 #endif
  703 
  704         inp = sotoinpcb(so);
  705         inp->inp_inc.inc_fibnum = so->so_fibnum;
  706         INP_WLOCK(inp);
  707         INP_HASH_WLOCK(&V_tcbinfo);
  708 
  709         /* Insert new socket into PCB hash list. */
  710         inp->inp_inc.inc_flags = sc->sc_inc.inc_flags;
  711 #ifdef INET6
  712         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
  713                 inp->in6p_laddr = sc->sc_inc.inc6_laddr;
  714         } else {
  715                 inp->inp_vflag &= ~INP_IPV6;
  716                 inp->inp_vflag |= INP_IPV4;
  717 #endif
  718                 inp->inp_laddr = sc->sc_inc.inc_laddr;
  719 #ifdef INET6
  720         }
  721 #endif
  722 
  723         /*
  724          * Install in the reservation hash table for now, but don't yet
  725          * install a connection group since the full 4-tuple isn't yet
  726          * configured.
  727          */
  728         inp->inp_lport = sc->sc_inc.inc_lport;
  729         if ((error = in_pcbinshash_nopcbgroup(inp)) != 0) {
  730                 /*
  731                  * Undo the assignments above if we failed to
  732                  * put the PCB on the hash lists.
  733                  */
  734 #ifdef INET6
  735                 if (sc->sc_inc.inc_flags & INC_ISIPV6)
  736                         inp->in6p_laddr = in6addr_any;
  737                 else
  738 #endif
  739                         inp->inp_laddr.s_addr = INADDR_ANY;
  740                 inp->inp_lport = 0;
  741                 if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  742                         log(LOG_DEBUG, "%s; %s: in_pcbinshash failed "
  743                             "with error %i\n",
  744                             s, __func__, error);
  745                         free(s, M_TCPLOG);
  746                 }
  747                 INP_HASH_WUNLOCK(&V_tcbinfo);
  748                 goto abort;
  749         }
  750 #ifdef IPSEC
  751         /* Copy old policy into new socket's. */
  752         if (ipsec_copy_policy(sotoinpcb(lso)->inp_sp, inp->inp_sp))
  753                 printf("syncache_socket: could not copy policy\n");
  754 #endif
  755 #ifdef INET6
  756         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
  757                 struct inpcb *oinp = sotoinpcb(lso);
  758                 struct in6_addr laddr6;
  759                 struct sockaddr_in6 sin6;
  760                 /*
  761                  * Inherit socket options from the listening socket.
  762                  * Note that in6p_inputopts are not (and should not be)
  763                  * copied, since it stores previously received options and is
  764                  * used to detect if each new option is different than the
  765                  * previous one and hence should be passed to a user.
  766                  * If we copied in6p_inputopts, a user would not be able to
  767                  * receive options just after calling the accept system call.
  768                  */
  769                 inp->inp_flags |= oinp->inp_flags & INP_CONTROLOPTS;
  770                 if (oinp->in6p_outputopts)
  771                         inp->in6p_outputopts =
  772                             ip6_copypktopts(oinp->in6p_outputopts, M_NOWAIT);
  773 
  774                 sin6.sin6_family = AF_INET6;
  775                 sin6.sin6_len = sizeof(sin6);
  776                 sin6.sin6_addr = sc->sc_inc.inc6_faddr;
  777                 sin6.sin6_port = sc->sc_inc.inc_fport;
  778                 sin6.sin6_flowinfo = sin6.sin6_scope_id = 0;
  779                 laddr6 = inp->in6p_laddr;
  780                 if (IN6_IS_ADDR_UNSPECIFIED(&inp->in6p_laddr))
  781                         inp->in6p_laddr = sc->sc_inc.inc6_laddr;
  782                 if ((error = in6_pcbconnect_mbuf(inp, (struct sockaddr *)&sin6,
  783                     thread0.td_ucred, m)) != 0) {
  784                         inp->in6p_laddr = laddr6;
  785                         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  786                                 log(LOG_DEBUG, "%s; %s: in6_pcbconnect failed "
  787                                     "with error %i\n",
  788                                     s, __func__, error);
  789                                 free(s, M_TCPLOG);
  790                         }
  791                         INP_HASH_WUNLOCK(&V_tcbinfo);
  792                         goto abort;
  793                 }
  794                 /* Override flowlabel from in6_pcbconnect. */
  795                 inp->inp_flow &= ~IPV6_FLOWLABEL_MASK;
  796                 inp->inp_flow |= sc->sc_flowlabel;
  797         }
  798 #endif /* INET6 */
  799 #if defined(INET) && defined(INET6)
  800         else
  801 #endif
  802 #ifdef INET
  803         {
  804                 struct in_addr laddr;
  805                 struct sockaddr_in sin;
  806 
  807                 inp->inp_options = (m) ? ip_srcroute(m) : NULL;
  808                 
  809                 if (inp->inp_options == NULL) {
  810                         inp->inp_options = sc->sc_ipopts;
  811                         sc->sc_ipopts = NULL;
  812                 }
  813 
  814                 sin.sin_family = AF_INET;
  815                 sin.sin_len = sizeof(sin);
  816                 sin.sin_addr = sc->sc_inc.inc_faddr;
  817                 sin.sin_port = sc->sc_inc.inc_fport;
  818                 bzero((caddr_t)sin.sin_zero, sizeof(sin.sin_zero));
  819                 laddr = inp->inp_laddr;
  820                 if (inp->inp_laddr.s_addr == INADDR_ANY)
  821                         inp->inp_laddr = sc->sc_inc.inc_laddr;
  822                 if ((error = in_pcbconnect_mbuf(inp, (struct sockaddr *)&sin,
  823                     thread0.td_ucred, m)) != 0) {
  824                         inp->inp_laddr = laddr;
  825                         if ((s = tcp_log_addrs(&sc->sc_inc, NULL, NULL, NULL))) {
  826                                 log(LOG_DEBUG, "%s; %s: in_pcbconnect failed "
  827                                     "with error %i\n",
  828                                     s, __func__, error);
  829                                 free(s, M_TCPLOG);
  830                         }
  831                         INP_HASH_WUNLOCK(&V_tcbinfo);
  832                         goto abort;
  833                 }
  834         }
  835 #endif /* INET */
  836         INP_HASH_WUNLOCK(&V_tcbinfo);
  837         tp = intotcpcb(inp);
  838         tcp_state_change(tp, TCPS_SYN_RECEIVED);
  839         tp->iss = sc->sc_iss;
  840         tp->irs = sc->sc_irs;
  841         tcp_rcvseqinit(tp);
  842         tcp_sendseqinit(tp);
  843         tp->snd_wl1 = sc->sc_irs;
  844         tp->snd_max = tp->iss + 1;
  845         tp->snd_nxt = tp->iss + 1;
  846         tp->rcv_up = sc->sc_irs + 1;
  847         tp->rcv_wnd = sc->sc_wnd;
  848         tp->rcv_adv += tp->rcv_wnd;
  849         tp->last_ack_sent = tp->rcv_nxt;
  850 
  851         tp->t_flags = sototcpcb(lso)->t_flags & (TF_NOPUSH|TF_NODELAY);
  852         if (sc->sc_flags & SCF_NOOPT)
  853                 tp->t_flags |= TF_NOOPT;
  854         else {
  855                 if (sc->sc_flags & SCF_WINSCALE) {
  856                         tp->t_flags |= TF_REQ_SCALE|TF_RCVD_SCALE;
  857                         tp->snd_scale = sc->sc_requested_s_scale;
  858                         tp->request_r_scale = sc->sc_requested_r_scale;
  859                 }
  860                 if (sc->sc_flags & SCF_TIMESTAMP) {
  861                         tp->t_flags |= TF_REQ_TSTMP|TF_RCVD_TSTMP;
  862                         tp->ts_recent = sc->sc_tsreflect;
  863                         tp->ts_recent_age = tcp_ts_getticks();
  864                         tp->ts_offset = sc->sc_tsoff;
  865                 }
  866 #ifdef TCP_SIGNATURE
  867                 if (sc->sc_flags & SCF_SIGNATURE)
  868                         tp->t_flags |= TF_SIGNATURE;
  869 #endif
  870                 if (sc->sc_flags & SCF_SACK)
  871                         tp->t_flags |= TF_SACK_PERMIT;
  872         }
  873 
  874         if (sc->sc_flags & SCF_ECN)
  875                 tp->t_flags |= TF_ECN_PERMIT;
  876 
  877         /*
  878          * Set up MSS and get cached values from tcp_hostcache.
  879          * This might overwrite some of the defaults we just set.
  880          */
  881         tcp_mss(tp, sc->sc_peer_mss);
  882 
  883         /*
  884          * If the SYN,ACK was retransmitted, indicate that CWND to be
  885          * limited to one segment in cc_conn_init().
  886          * NB: sc_rxmits counts all SYN,ACK transmits, not just retransmits.
  887          */
  888         if (sc->sc_rxmits > 1)
  889                 tp->snd_cwnd = 1;
  890 
  891 #ifdef TCP_OFFLOAD
  892         /*
  893          * Allow a TOE driver to install its hooks.  Note that we hold the
  894          * pcbinfo lock too and that prevents tcp_usr_accept from accepting a
  895          * new connection before the TOE driver has done its thing.
  896          */
  897         if (ADDED_BY_TOE(sc)) {
  898                 struct toedev *tod = sc->sc_tod;
  899 
  900                 tod->tod_offload_socket(tod, sc->sc_todctx, so);
  901         }
  902 #endif
  903         /*
  904          * Copy and activate timers.
  905          */
  906         tp->t_keepinit = sototcpcb(lso)->t_keepinit;
  907         tp->t_keepidle = sototcpcb(lso)->t_keepidle;
  908         tp->t_keepintvl = sototcpcb(lso)->t_keepintvl;
  909         tp->t_keepcnt = sototcpcb(lso)->t_keepcnt;
  910         tcp_timer_activate(tp, TT_KEEP, TP_KEEPINIT(tp));
  911 
  912         INP_WUNLOCK(inp);
  913 
  914         TCPSTAT_INC(tcps_accepts);
  915         return (so);
  916 
  917 abort:
  918         INP_WUNLOCK(inp);
  919 abort2:
  920         if (so != NULL)
  921                 soabort(so);
  922         return (NULL);
  923 }
  924 
  925 /*
  926  * This function gets called when we receive an ACK for a
  927  * socket in the LISTEN state.  We look up the connection
  928  * in the syncache, and if its there, we pull it out of
  929  * the cache and turn it into a full-blown connection in
  930  * the SYN-RECEIVED state.
  931  */
  932 int
  933 syncache_expand(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
  934     struct socket **lsop, struct mbuf *m)
  935 {
  936         struct syncache *sc;
  937         struct syncache_head *sch;
  938         struct syncache scs;
  939         char *s;
  940 
  941         /*
  942          * Global TCP locks are held because we manipulate the PCB lists
  943          * and create a new socket.
  944          */
  945         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
  946         KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_ACK,
  947             ("%s: can handle only ACK", __func__));
  948 
  949         sc = syncache_lookup(inc, &sch);        /* returns locked sch */
  950         SCH_LOCK_ASSERT(sch);
  951 
  952 #ifdef INVARIANTS
  953         /*
  954          * Test code for syncookies comparing the syncache stored
  955          * values with the reconstructed values from the cookie.
  956          */
  957         if (sc != NULL)
  958                 syncookie_cmp(inc, sch, sc, th, to, *lsop);
  959 #endif
  960 
  961         if (sc == NULL) {
  962                 /*
  963                  * There is no syncache entry, so see if this ACK is
  964                  * a returning syncookie.  To do this, first:
  965                  *  A. See if this socket has had a syncache entry dropped in
  966                  *     the past.  We don't want to accept a bogus syncookie
  967                  *     if we've never received a SYN.
  968                  *  B. check that the syncookie is valid.  If it is, then
  969                  *     cobble up a fake syncache entry, and return.
  970                  */
  971                 if (!V_tcp_syncookies) {
  972                         SCH_UNLOCK(sch);
  973                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  974                                 log(LOG_DEBUG, "%s; %s: Spurious ACK, "
  975                                     "segment rejected (syncookies disabled)\n",
  976                                     s, __func__);
  977                         goto failed;
  978                 }
  979                 bzero(&scs, sizeof(scs));
  980                 sc = syncookie_lookup(inc, sch, &scs, th, to, *lsop);
  981                 SCH_UNLOCK(sch);
  982                 if (sc == NULL) {
  983                         if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
  984                                 log(LOG_DEBUG, "%s; %s: Segment failed "
  985                                     "SYNCOOKIE authentication, segment rejected "
  986                                     "(probably spoofed)\n", s, __func__);
  987                         goto failed;
  988                 }
  989         } else {
  990                 /* Pull out the entry to unlock the bucket row. */
  991                 TAILQ_REMOVE(&sch->sch_bucket, sc, sc_hash);
  992                 sch->sch_length--;
  993 #ifdef TCP_OFFLOAD
  994                 if (ADDED_BY_TOE(sc)) {
  995                         struct toedev *tod = sc->sc_tod;
  996 
  997                         tod->tod_syncache_removed(tod, sc->sc_todctx);
  998                 }
  999 #endif
 1000                 SCH_UNLOCK(sch);
 1001         }
 1002 
 1003         /*
 1004          * Segment validation:
 1005          * ACK must match our initial sequence number + 1 (the SYN|ACK).
 1006          */
 1007         if (th->th_ack != sc->sc_iss + 1) {
 1008                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1009                         log(LOG_DEBUG, "%s; %s: ACK %u != ISS+1 %u, segment "
 1010                             "rejected\n", s, __func__, th->th_ack, sc->sc_iss);
 1011                 goto failed;
 1012         }
 1013 
 1014         /*
 1015          * The SEQ must fall in the window starting at the received
 1016          * initial receive sequence number + 1 (the SYN).
 1017          */
 1018         if (SEQ_LEQ(th->th_seq, sc->sc_irs) ||
 1019             SEQ_GT(th->th_seq, sc->sc_irs + sc->sc_wnd)) {
 1020                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1021                         log(LOG_DEBUG, "%s; %s: SEQ %u != IRS+1 %u, segment "
 1022                             "rejected\n", s, __func__, th->th_seq, sc->sc_irs);
 1023                 goto failed;
 1024         }
 1025 
 1026         /*
 1027          * If timestamps were not negotiated during SYN/ACK they
 1028          * must not appear on any segment during this session.
 1029          */
 1030         if (!(sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS)) {
 1031                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1032                         log(LOG_DEBUG, "%s; %s: Timestamp not expected, "
 1033                             "segment rejected\n", s, __func__);
 1034                 goto failed;
 1035         }
 1036 
 1037         /*
 1038          * If timestamps were negotiated during SYN/ACK they should
 1039          * appear on every segment during this session.
 1040          * XXXAO: This is only informal as there have been unverified
 1041          * reports of non-compliants stacks.
 1042          */
 1043         if ((sc->sc_flags & SCF_TIMESTAMP) && !(to->to_flags & TOF_TS)) {
 1044                 if ((s = tcp_log_addrs(inc, th, NULL, NULL))) {
 1045                         log(LOG_DEBUG, "%s; %s: Timestamp missing, "
 1046                             "no action\n", s, __func__);
 1047                         free(s, M_TCPLOG);
 1048                         s = NULL;
 1049                 }
 1050         }
 1051 
 1052         /*
 1053          * If timestamps were negotiated the reflected timestamp
 1054          * must be equal to what we actually sent in the SYN|ACK.
 1055          */
 1056         if ((to->to_flags & TOF_TS) && to->to_tsecr != sc->sc_ts) {
 1057                 if ((s = tcp_log_addrs(inc, th, NULL, NULL)))
 1058                         log(LOG_DEBUG, "%s; %s: TSECR %u != TS %u, "
 1059                             "segment rejected\n",
 1060                             s, __func__, to->to_tsecr, sc->sc_ts);
 1061                 goto failed;
 1062         }
 1063 
 1064         *lsop = syncache_socket(sc, *lsop, m);
 1065 
 1066         if (*lsop == NULL)
 1067                 TCPSTAT_INC(tcps_sc_aborted);
 1068         else
 1069                 TCPSTAT_INC(tcps_sc_completed);
 1070 
 1071 /* how do we find the inp for the new socket? */
 1072         if (sc != &scs)
 1073                 syncache_free(sc);
 1074         return (1);
 1075 failed:
 1076         if (sc != NULL && sc != &scs)
 1077                 syncache_free(sc);
 1078         if (s != NULL)
 1079                 free(s, M_TCPLOG);
 1080         *lsop = NULL;
 1081         return (0);
 1082 }
 1083 
 1084 /*
 1085  * Given a LISTEN socket and an inbound SYN request, add
 1086  * this to the syn cache, and send back a segment:
 1087  *      <SEQ=ISS><ACK=RCV_NXT><CTL=SYN,ACK>
 1088  * to the source.
 1089  *
 1090  * IMPORTANT NOTE: We do _NOT_ ACK data that might accompany the SYN.
 1091  * Doing so would require that we hold onto the data and deliver it
 1092  * to the application.  However, if we are the target of a SYN-flood
 1093  * DoS attack, an attacker could send data which would eventually
 1094  * consume all available buffer space if it were ACKed.  By not ACKing
 1095  * the data, we avoid this DoS scenario.
 1096  */
 1097 void
 1098 syncache_add(struct in_conninfo *inc, struct tcpopt *to, struct tcphdr *th,
 1099     struct inpcb *inp, struct socket **lsop, struct mbuf *m, void *tod,
 1100     void *todctx)
 1101 {
 1102         struct tcpcb *tp;
 1103         struct socket *so;
 1104         struct syncache *sc = NULL;
 1105         struct syncache_head *sch;
 1106         struct mbuf *ipopts = NULL;
 1107         u_int ltflags;
 1108         int win, sb_hiwat, ip_ttl, ip_tos;
 1109         char *s;
 1110 #ifdef INET6
 1111         int autoflowlabel = 0;
 1112 #endif
 1113 #ifdef MAC
 1114         struct label *maclabel;
 1115 #endif
 1116         struct syncache scs;
 1117         struct ucred *cred;
 1118 
 1119         INP_INFO_WLOCK_ASSERT(&V_tcbinfo);
 1120         INP_WLOCK_ASSERT(inp);                  /* listen socket */
 1121         KASSERT((th->th_flags & (TH_RST|TH_ACK|TH_SYN)) == TH_SYN,
 1122             ("%s: unexpected tcp flags", __func__));
 1123 
 1124         /*
 1125          * Combine all so/tp operations very early to drop the INP lock as
 1126          * soon as possible.
 1127          */
 1128         so = *lsop;
 1129         tp = sototcpcb(so);
 1130         cred = crhold(so->so_cred);
 1131 
 1132 #ifdef INET6
 1133         if ((inc->inc_flags & INC_ISIPV6) &&
 1134             (inp->inp_flags & IN6P_AUTOFLOWLABEL))
 1135                 autoflowlabel = 1;
 1136 #endif
 1137         ip_ttl = inp->inp_ip_ttl;
 1138         ip_tos = inp->inp_ip_tos;
 1139         win = sbspace(&so->so_rcv);
 1140         sb_hiwat = so->so_rcv.sb_hiwat;
 1141         ltflags = (tp->t_flags & (TF_NOOPT | TF_SIGNATURE));
 1142 
 1143         /* By the time we drop the lock these should no longer be used. */
 1144         so = NULL;
 1145         tp = NULL;
 1146 
 1147 #ifdef MAC
 1148         if (mac_syncache_init(&maclabel) != 0) {
 1149                 INP_WUNLOCK(inp);
 1150                 INP_INFO_WUNLOCK(&V_tcbinfo);
 1151                 goto done;
 1152         } else
 1153                 mac_syncache_create(maclabel, inp);
 1154 #endif
 1155         INP_WUNLOCK(inp);
 1156         INP_INFO_WUNLOCK(&V_tcbinfo);
 1157 
 1158         /*
 1159          * Remember the IP options, if any.
 1160          */
 1161 #ifdef INET6
 1162         if (!(inc->inc_flags & INC_ISIPV6))
 1163 #endif
 1164 #ifdef INET
 1165                 ipopts = (m) ? ip_srcroute(m) : NULL;
 1166 #else
 1167                 ipopts = NULL;
 1168 #endif
 1169 
 1170         /*
 1171          * See if we already have an entry for this connection.
 1172          * If we do, resend the SYN,ACK, and reset the retransmit timer.
 1173          *
 1174          * XXX: should the syncache be re-initialized with the contents
 1175          * of the new SYN here (which may have different options?)
 1176          *
 1177          * XXX: We do not check the sequence number to see if this is a
 1178          * real retransmit or a new connection attempt.  The question is
 1179          * how to handle such a case; either ignore it as spoofed, or
 1180          * drop the current entry and create a new one?
 1181          */
 1182         sc = syncache_lookup(inc, &sch);        /* returns locked entry */
 1183         SCH_LOCK_ASSERT(sch);
 1184         if (sc != NULL) {
 1185                 TCPSTAT_INC(tcps_sc_dupsyn);
 1186                 if (ipopts) {
 1187                         /*
 1188                          * If we were remembering a previous source route,
 1189                          * forget it and use the new one we've been given.
 1190                          */
 1191                         if (sc->sc_ipopts)
 1192                                 (void) m_free(sc->sc_ipopts);
 1193                         sc->sc_ipopts = ipopts;
 1194                 }
 1195                 /*
 1196                  * Update timestamp if present.
 1197                  */
 1198                 if ((sc->sc_flags & SCF_TIMESTAMP) && (to->to_flags & TOF_TS))
 1199                         sc->sc_tsreflect = to->to_tsval;
 1200                 else
 1201                         sc->sc_flags &= ~SCF_TIMESTAMP;
 1202 #ifdef MAC
 1203                 /*
 1204                  * Since we have already unconditionally allocated label
 1205                  * storage, free it up.  The syncache entry will already
 1206                  * have an initialized label we can use.
 1207                  */
 1208                 mac_syncache_destroy(&maclabel);
 1209 #endif
 1210                 /* Retransmit SYN|ACK and reset retransmit count. */
 1211                 if ((s = tcp_log_addrs(&sc->sc_inc, th, NULL, NULL))) {
 1212                         log(LOG_DEBUG, "%s; %s: Received duplicate SYN, "
 1213                             "resetting timer and retransmitting SYN|ACK\n",
 1214                             s, __func__);
 1215                         free(s, M_TCPLOG);
 1216                 }
 1217                 if (syncache_respond(sc) == 0) {
 1218                         sc->sc_rxmits = 0;
 1219                         syncache_timeout(sc, sch, 1);
 1220                         TCPSTAT_INC(tcps_sndacks);
 1221                         TCPSTAT_INC(tcps_sndtotal);
 1222                 }
 1223                 SCH_UNLOCK(sch);
 1224                 goto done;
 1225         }
 1226 
 1227         sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
 1228         if (sc == NULL) {
 1229                 /*
 1230                  * The zone allocator couldn't provide more entries.
 1231                  * Treat this as if the cache was full; drop the oldest
 1232                  * entry and insert the new one.
 1233                  */
 1234                 TCPSTAT_INC(tcps_sc_zonefail);
 1235                 if ((sc = TAILQ_LAST(&sch->sch_bucket, sch_head)) != NULL)
 1236                         syncache_drop(sc, sch);
 1237                 sc = uma_zalloc(V_tcp_syncache.zone, M_NOWAIT | M_ZERO);
 1238                 if (sc == NULL) {
 1239                         if (V_tcp_syncookies) {
 1240                                 bzero(&scs, sizeof(scs));
 1241                                 sc = &scs;
 1242                         } else {
 1243                                 SCH_UNLOCK(sch);
 1244                                 if (ipopts)
 1245                                         (void) m_free(ipopts);
 1246                                 goto done;
 1247                         }
 1248                 }
 1249         }
 1250         
 1251         /*
 1252          * Fill in the syncache values.
 1253          */
 1254 #ifdef MAC
 1255         sc->sc_label = maclabel;
 1256 #endif
 1257         sc->sc_cred = cred;
 1258         cred = NULL;
 1259         sc->sc_ipopts = ipopts;
 1260         bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
 1261 #ifdef INET6
 1262         if (!(inc->inc_flags & INC_ISIPV6))
 1263 #endif
 1264         {
 1265                 sc->sc_ip_tos = ip_tos;
 1266                 sc->sc_ip_ttl = ip_ttl;
 1267         }
 1268 #ifdef TCP_OFFLOAD
 1269         sc->sc_tod = tod;
 1270         sc->sc_todctx = todctx;
 1271 #endif
 1272         sc->sc_irs = th->th_seq;
 1273         sc->sc_iss = arc4random();
 1274         sc->sc_flags = 0;
 1275         sc->sc_flowlabel = 0;
 1276 
 1277         /*
 1278          * Initial receive window: clip sbspace to [0 .. TCP_MAXWIN].
 1279          * win was derived from socket earlier in the function.
 1280          */
 1281         win = imax(win, 0);
 1282         win = imin(win, TCP_MAXWIN);
 1283         sc->sc_wnd = win;
 1284 
 1285         if (V_tcp_do_rfc1323) {
 1286                 /*
 1287                  * A timestamp received in a SYN makes
 1288                  * it ok to send timestamp requests and replies.
 1289                  */
 1290                 if (to->to_flags & TOF_TS) {
 1291                         sc->sc_tsreflect = to->to_tsval;
 1292                         sc->sc_ts = tcp_ts_getticks();
 1293                         sc->sc_flags |= SCF_TIMESTAMP;
 1294                 }
 1295                 if (to->to_flags & TOF_SCALE) {
 1296                         int wscale = 0;
 1297 
 1298                         /*
 1299                          * Pick the smallest possible scaling factor that
 1300                          * will still allow us to scale up to sb_max, aka
 1301                          * kern.ipc.maxsockbuf.
 1302                          *
 1303                          * We do this because there are broken firewalls that
 1304                          * will corrupt the window scale option, leading to
 1305                          * the other endpoint believing that our advertised
 1306                          * window is unscaled.  At scale factors larger than
 1307                          * 5 the unscaled window will drop below 1500 bytes,
 1308                          * leading to serious problems when traversing these
 1309                          * broken firewalls.
 1310                          *
 1311                          * With the default maxsockbuf of 256K, a scale factor
 1312                          * of 3 will be chosen by this algorithm.  Those who
 1313                          * choose a larger maxsockbuf should watch out
 1314                          * for the compatiblity problems mentioned above.
 1315                          *
 1316                          * RFC1323: The Window field in a SYN (i.e., a <SYN>
 1317                          * or <SYN,ACK>) segment itself is never scaled.
 1318                          */
 1319                         while (wscale < TCP_MAX_WINSHIFT &&
 1320                             (TCP_MAXWIN << wscale) < sb_max)
 1321                                 wscale++;
 1322                         sc->sc_requested_r_scale = wscale;
 1323                         sc->sc_requested_s_scale = to->to_wscale;
 1324                         sc->sc_flags |= SCF_WINSCALE;
 1325                 }
 1326         }
 1327 #ifdef TCP_SIGNATURE
 1328         /*
 1329          * If listening socket requested TCP digests, and received SYN
 1330          * contains the option, flag this in the syncache so that
 1331          * syncache_respond() will do the right thing with the SYN+ACK.
 1332          * XXX: Currently we always record the option by default and will
 1333          * attempt to use it in syncache_respond().
 1334          */
 1335         if (to->to_flags & TOF_SIGNATURE || ltflags & TF_SIGNATURE)
 1336                 sc->sc_flags |= SCF_SIGNATURE;
 1337 #endif
 1338         if (to->to_flags & TOF_SACKPERM)
 1339                 sc->sc_flags |= SCF_SACK;
 1340         if (to->to_flags & TOF_MSS)
 1341                 sc->sc_peer_mss = to->to_mss;   /* peer mss may be zero */
 1342         if (ltflags & TF_NOOPT)
 1343                 sc->sc_flags |= SCF_NOOPT;
 1344         if ((th->th_flags & (TH_ECE|TH_CWR)) && V_tcp_do_ecn)
 1345                 sc->sc_flags |= SCF_ECN;
 1346 
 1347         if (V_tcp_syncookies)
 1348                 sc->sc_iss = syncookie_generate(sch, sc);
 1349 #ifdef INET6
 1350         if (autoflowlabel) {
 1351                 if (V_tcp_syncookies)
 1352                         sc->sc_flowlabel = sc->sc_iss;
 1353                 else
 1354                         sc->sc_flowlabel = ip6_randomflowlabel();
 1355                 sc->sc_flowlabel = htonl(sc->sc_flowlabel) & IPV6_FLOWLABEL_MASK;
 1356         }
 1357 #endif
 1358         SCH_UNLOCK(sch);
 1359 
 1360         /*
 1361          * Do a standard 3-way handshake.
 1362          */
 1363         if (syncache_respond(sc) == 0) {
 1364                 if (V_tcp_syncookies && V_tcp_syncookiesonly && sc != &scs)
 1365                         syncache_free(sc);
 1366                 else if (sc != &scs)
 1367                         syncache_insert(sc, sch);   /* locks and unlocks sch */
 1368                 TCPSTAT_INC(tcps_sndacks);
 1369                 TCPSTAT_INC(tcps_sndtotal);
 1370         } else {
 1371                 if (sc != &scs)
 1372                         syncache_free(sc);
 1373                 TCPSTAT_INC(tcps_sc_dropped);
 1374         }
 1375 
 1376 done:
 1377         if (cred != NULL)
 1378                 crfree(cred);
 1379 #ifdef MAC
 1380         if (sc == &scs)
 1381                 mac_syncache_destroy(&maclabel);
 1382 #endif
 1383         if (m) {
 1384                 
 1385                 *lsop = NULL;
 1386                 m_freem(m);
 1387         }
 1388 }
 1389 
 1390 static int
 1391 syncache_respond(struct syncache *sc)
 1392 {
 1393         struct ip *ip = NULL;
 1394         struct mbuf *m;
 1395         struct tcphdr *th = NULL;
 1396         int optlen, error = 0;  /* Make compiler happy */
 1397         u_int16_t hlen, tlen, mssopt;
 1398         struct tcpopt to;
 1399 #ifdef INET6
 1400         struct ip6_hdr *ip6 = NULL;
 1401 #endif
 1402 
 1403         hlen =
 1404 #ifdef INET6
 1405                (sc->sc_inc.inc_flags & INC_ISIPV6) ? sizeof(struct ip6_hdr) :
 1406 #endif
 1407                 sizeof(struct ip);
 1408         tlen = hlen + sizeof(struct tcphdr);
 1409 
 1410         /* Determine MSS we advertize to other end of connection. */
 1411         mssopt = tcp_mssopt(&sc->sc_inc);
 1412         if (sc->sc_peer_mss)
 1413                 mssopt = max( min(sc->sc_peer_mss, mssopt), V_tcp_minmss);
 1414 
 1415         /* XXX: Assume that the entire packet will fit in a header mbuf. */
 1416         KASSERT(max_linkhdr + tlen + TCP_MAXOLEN <= MHLEN,
 1417             ("syncache: mbuf too small"));
 1418 
 1419         /* Create the IP+TCP header from scratch. */
 1420         m = m_gethdr(M_NOWAIT, MT_DATA);
 1421         if (m == NULL)
 1422                 return (ENOBUFS);
 1423 #ifdef MAC
 1424         mac_syncache_create_mbuf(sc->sc_label, m);
 1425 #endif
 1426         m->m_data += max_linkhdr;
 1427         m->m_len = tlen;
 1428         m->m_pkthdr.len = tlen;
 1429         m->m_pkthdr.rcvif = NULL;
 1430 
 1431 #ifdef INET6
 1432         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
 1433                 ip6 = mtod(m, struct ip6_hdr *);
 1434                 ip6->ip6_vfc = IPV6_VERSION;
 1435                 ip6->ip6_nxt = IPPROTO_TCP;
 1436                 ip6->ip6_src = sc->sc_inc.inc6_laddr;
 1437                 ip6->ip6_dst = sc->sc_inc.inc6_faddr;
 1438                 ip6->ip6_plen = htons(tlen - hlen);
 1439                 /* ip6_hlim is set after checksum */
 1440                 ip6->ip6_flow &= ~IPV6_FLOWLABEL_MASK;
 1441                 ip6->ip6_flow |= sc->sc_flowlabel;
 1442 
 1443                 th = (struct tcphdr *)(ip6 + 1);
 1444         }
 1445 #endif
 1446 #if defined(INET6) && defined(INET)
 1447         else
 1448 #endif
 1449 #ifdef INET
 1450         {
 1451                 ip = mtod(m, struct ip *);
 1452                 ip->ip_v = IPVERSION;
 1453                 ip->ip_hl = sizeof(struct ip) >> 2;
 1454                 ip->ip_len = htons(tlen);
 1455                 ip->ip_id = 0;
 1456                 ip->ip_off = 0;
 1457                 ip->ip_sum = 0;
 1458                 ip->ip_p = IPPROTO_TCP;
 1459                 ip->ip_src = sc->sc_inc.inc_laddr;
 1460                 ip->ip_dst = sc->sc_inc.inc_faddr;
 1461                 ip->ip_ttl = sc->sc_ip_ttl;
 1462                 ip->ip_tos = sc->sc_ip_tos;
 1463 
 1464                 /*
 1465                  * See if we should do MTU discovery.  Route lookups are
 1466                  * expensive, so we will only unset the DF bit if:
 1467                  *
 1468                  *      1) path_mtu_discovery is disabled
 1469                  *      2) the SCF_UNREACH flag has been set
 1470                  */
 1471                 if (V_path_mtu_discovery && ((sc->sc_flags & SCF_UNREACH) == 0))
 1472                        ip->ip_off |= htons(IP_DF);
 1473 
 1474                 th = (struct tcphdr *)(ip + 1);
 1475         }
 1476 #endif /* INET */
 1477         th->th_sport = sc->sc_inc.inc_lport;
 1478         th->th_dport = sc->sc_inc.inc_fport;
 1479 
 1480         th->th_seq = htonl(sc->sc_iss);
 1481         th->th_ack = htonl(sc->sc_irs + 1);
 1482         th->th_off = sizeof(struct tcphdr) >> 2;
 1483         th->th_x2 = 0;
 1484         th->th_flags = TH_SYN|TH_ACK;
 1485         th->th_win = htons(sc->sc_wnd);
 1486         th->th_urp = 0;
 1487 
 1488         if (sc->sc_flags & SCF_ECN) {
 1489                 th->th_flags |= TH_ECE;
 1490                 TCPSTAT_INC(tcps_ecn_shs);
 1491         }
 1492 
 1493         /* Tack on the TCP options. */
 1494         if ((sc->sc_flags & SCF_NOOPT) == 0) {
 1495                 to.to_flags = 0;
 1496 
 1497                 to.to_mss = mssopt;
 1498                 to.to_flags = TOF_MSS;
 1499                 if (sc->sc_flags & SCF_WINSCALE) {
 1500                         to.to_wscale = sc->sc_requested_r_scale;
 1501                         to.to_flags |= TOF_SCALE;
 1502                 }
 1503                 if (sc->sc_flags & SCF_TIMESTAMP) {
 1504                         /* Virgin timestamp or TCP cookie enhanced one. */
 1505                         to.to_tsval = sc->sc_ts;
 1506                         to.to_tsecr = sc->sc_tsreflect;
 1507                         to.to_flags |= TOF_TS;
 1508                 }
 1509                 if (sc->sc_flags & SCF_SACK)
 1510                         to.to_flags |= TOF_SACKPERM;
 1511 #ifdef TCP_SIGNATURE
 1512                 if (sc->sc_flags & SCF_SIGNATURE)
 1513                         to.to_flags |= TOF_SIGNATURE;
 1514 #endif
 1515                 optlen = tcp_addoptions(&to, (u_char *)(th + 1));
 1516 
 1517                 /* Adjust headers by option size. */
 1518                 th->th_off = (sizeof(struct tcphdr) + optlen) >> 2;
 1519                 m->m_len += optlen;
 1520                 m->m_pkthdr.len += optlen;
 1521 
 1522 #ifdef TCP_SIGNATURE
 1523                 if (sc->sc_flags & SCF_SIGNATURE)
 1524                         tcp_signature_compute(m, 0, 0, optlen,
 1525                             to.to_signature, IPSEC_DIR_OUTBOUND);
 1526 #endif
 1527 #ifdef INET6
 1528                 if (sc->sc_inc.inc_flags & INC_ISIPV6)
 1529                         ip6->ip6_plen = htons(ntohs(ip6->ip6_plen) + optlen);
 1530                 else
 1531 #endif
 1532                         ip->ip_len = htons(ntohs(ip->ip_len) + optlen);
 1533         } else
 1534                 optlen = 0;
 1535 
 1536         M_SETFIB(m, sc->sc_inc.inc_fibnum);
 1537         m->m_pkthdr.csum_data = offsetof(struct tcphdr, th_sum);
 1538 #ifdef INET6
 1539         if (sc->sc_inc.inc_flags & INC_ISIPV6) {
 1540                 m->m_pkthdr.csum_flags = CSUM_TCP_IPV6;
 1541                 th->th_sum = in6_cksum_pseudo(ip6, tlen + optlen - hlen,
 1542                     IPPROTO_TCP, 0);
 1543                 ip6->ip6_hlim = in6_selecthlim(NULL, NULL);
 1544 #ifdef TCP_OFFLOAD
 1545                 if (ADDED_BY_TOE(sc)) {
 1546                         struct toedev *tod = sc->sc_tod;
 1547 
 1548                         error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
 1549 
 1550                         return (error);
 1551                 }
 1552 #endif
 1553                 error = ip6_output(m, NULL, NULL, 0, NULL, NULL, NULL);
 1554         }
 1555 #endif
 1556 #if defined(INET6) && defined(INET)
 1557         else
 1558 #endif
 1559 #ifdef INET
 1560         {
 1561                 m->m_pkthdr.csum_flags = CSUM_TCP;
 1562                 th->th_sum = in_pseudo(ip->ip_src.s_addr, ip->ip_dst.s_addr,
 1563                     htons(tlen + optlen - hlen + IPPROTO_TCP));
 1564 #ifdef TCP_OFFLOAD
 1565                 if (ADDED_BY_TOE(sc)) {
 1566                         struct toedev *tod = sc->sc_tod;
 1567 
 1568                         error = tod->tod_syncache_respond(tod, sc->sc_todctx, m);
 1569 
 1570                         return (error);
 1571                 }
 1572 #endif
 1573                 error = ip_output(m, sc->sc_ipopts, NULL, 0, NULL, NULL);
 1574         }
 1575 #endif
 1576         return (error);
 1577 }
 1578 
 1579 /*
 1580  * The purpose of syncookies is to handle spoofed SYN flooding DoS attacks
 1581  * that exceed the capacity of the syncache by avoiding the storage of any
 1582  * of the SYNs we receive.  Syncookies defend against blind SYN flooding
 1583  * attacks where the attacker does not have access to our responses.
 1584  *
 1585  * Syncookies encode and include all necessary information about the
 1586  * connection setup within the SYN|ACK that we send back.  That way we
 1587  * can avoid keeping any local state until the ACK to our SYN|ACK returns
 1588  * (if ever).  Normally the syncache and syncookies are running in parallel
 1589  * with the latter taking over when the former is exhausted.  When matching
 1590  * syncache entry is found the syncookie is ignored.
 1591  *
 1592  * The only reliable information persisting the 3WHS is our inital sequence
 1593  * number ISS of 32 bits.  Syncookies embed a cryptographically sufficient
 1594  * strong hash (MAC) value and a few bits of TCP SYN options in the ISS
 1595  * of our SYN|ACK.  The MAC can be recomputed when the ACK to our SYN|ACK
 1596  * returns and signifies a legitimate connection if it matches the ACK.
 1597  *
 1598  * The available space of 32 bits to store the hash and to encode the SYN
 1599  * option information is very tight and we should have at least 24 bits for
 1600  * the MAC to keep the number of guesses by blind spoofing reasonably high.
 1601  *
 1602  * SYN option information we have to encode to fully restore a connection:
 1603  * MSS: is imporant to chose an optimal segment size to avoid IP level
 1604  *   fragmentation along the path.  The common MSS values can be encoded
 1605  *   in a 3-bit table.  Uncommon values are captured by the next lower value
 1606  *   in the table leading to a slight increase in packetization overhead.
 1607  * WSCALE: is necessary to allow large windows to be used for high delay-
 1608  *   bandwidth product links.  Not scaling the window when it was initially
 1609  *   negotiated is bad for performance as lack of scaling further decreases
 1610  *   the apparent available send window.  We only need to encode the WSCALE
 1611  *   we received from the remote end.  Our end can be recalculated at any
 1612  *   time.  The common WSCALE values can be encoded in a 3-bit table.
 1613  *   Uncommon values are captured by the next lower value in the table
 1614  *   making us under-estimate the available window size halving our
 1615  *   theoretically possible maximum throughput for that connection.
 1616  * SACK: Greatly assists in packet loss recovery and requires 1 bit.
 1617  * TIMESTAMP and SIGNATURE is not encoded because they are permanent options
 1618  *   that are included in all segments on a connection.  We enable them when
 1619  *   the ACK has them.
 1620  *
 1621  * Security of syncookies and attack vectors:
 1622  *
 1623  * The MAC is computed over (faddr||laddr||fport||lport||irs||flags||secmod)
 1624  * together with the gloabl secret to make it unique per connection attempt.
 1625  * Thus any change of any of those parameters results in a different MAC output
 1626  * in an unpredictable way unless a collision is encountered.  24 bits of the
 1627  * MAC are embedded into the ISS.
 1628  *
 1629  * To prevent replay attacks two rotating global secrets are updated with a
 1630  * new random value every 15 seconds.  The life-time of a syncookie is thus
 1631  * 15-30 seconds.
 1632  *
 1633  * Vector 1: Attacking the secret.  This requires finding a weakness in the
 1634  * MAC itself or the way it is used here.  The attacker can do a chosen plain
 1635  * text attack by varying and testing the all parameters under his control.
 1636  * The strength depends on the size and randomness of the secret, and the
 1637  * cryptographic security of the MAC function.  Due to the constant updating
 1638  * of the secret the attacker has at most 29.999 seconds to find the secret
 1639  * and launch spoofed connections.  After that he has to start all over again.
 1640  *
 1641  * Vector 2: Collision attack on the MAC of a single ACK.  With a 24 bit MAC
 1642  * size an average of 4,823 attempts are required for a 50% chance of success
 1643  * to spoof a single syncookie (birthday collision paradox).  However the
 1644  * attacker is blind and doesn't know if one of his attempts succeeded unless
 1645  * he has a side channel to interfere success from.  A single connection setup
 1646  * success average of 90% requires 8,790 packets, 99.99% requires 17,578 packets.
 1647  * This many attempts are required for each one blind spoofed connection.  For
 1648  * every additional spoofed connection he has to launch another N attempts.
 1649  * Thus for a sustained rate 100 spoofed connections per second approximately
 1650  * 1,800,000 packets per second would have to be sent.
 1651  *
 1652  * NB: The MAC function should be fast so that it doesn't become a CPU
 1653  * exhaustion attack vector itself.
 1654  *
 1655  * References:
 1656  *  RFC4987 TCP SYN Flooding Attacks and Common Mitigations
 1657  *  SYN cookies were first proposed by cryptographer Dan J. Bernstein in 1996
 1658  *   http://cr.yp.to/syncookies.html    (overview)
 1659  *   http://cr.yp.to/syncookies/archive (details)
 1660  *
 1661  *
 1662  * Schematic construction of a syncookie enabled Initial Sequence Number:
 1663  *  0        1         2         3
 1664  *  12345678901234567890123456789012
 1665  * |xxxxxxxxxxxxxxxxxxxxxxxxWWWMMMSP|
 1666  *
 1667  *  x 24 MAC (truncated)
 1668  *  W  3 Send Window Scale index
 1669  *  M  3 MSS index
 1670  *  S  1 SACK permitted
 1671  *  P  1 Odd/even secret
 1672  */
 1673 
 1674 /*
 1675  * Distribution and probability of certain MSS values.  Those in between are
 1676  * rounded down to the next lower one.
 1677  * [An Analysis of TCP Maximum Segment Sizes, S. Alcock and R. Nelson, 2011]
 1678  *                            .2%  .3%   5%    7%    7%    20%   15%   45%
 1679  */
 1680 static int tcp_sc_msstab[] = { 216, 536, 1200, 1360, 1400, 1440, 1452, 1460 };
 1681 
 1682 /*
 1683  * Distribution and probability of certain WSCALE values.  We have to map the
 1684  * (send) window scale (shift) option with a range of 0-14 from 4 bits into 3
 1685  * bits based on prevalence of certain values.  Where we don't have an exact
 1686  * match for are rounded down to the next lower one letting us under-estimate
 1687  * the true available window.  At the moment this would happen only for the
 1688  * very uncommon values 3, 5 and those above 8 (more than 16MB socket buffer
 1689  * and window size).  The absence of the WSCALE option (no scaling in either
 1690  * direction) is encoded with index zero.
 1691  * [WSCALE values histograms, Allman, 2012]
 1692  *                            X 10 10 35  5  6 14 10%   by host
 1693  *                            X 11  4  5  5 18 49  3%   by connections
 1694  */
 1695 static int tcp_sc_wstab[] = { 0, 0, 1, 2, 4, 6, 7, 8 };
 1696 
 1697 /*
 1698  * Compute the MAC for the SYN cookie.  SIPHASH-2-4 is chosen for its speed
 1699  * and good cryptographic properties.
 1700  */
 1701 static uint32_t
 1702 syncookie_mac(struct in_conninfo *inc, tcp_seq irs, uint8_t flags,
 1703     uint8_t *secbits, uintptr_t secmod)
 1704 {
 1705         SIPHASH_CTX ctx;
 1706         uint32_t siphash[2];
 1707 
 1708         SipHash24_Init(&ctx);
 1709         SipHash_SetKey(&ctx, secbits);
 1710         switch (inc->inc_flags & INC_ISIPV6) {
 1711 #ifdef INET
 1712         case 0:
 1713                 SipHash_Update(&ctx, &inc->inc_faddr, sizeof(inc->inc_faddr));
 1714                 SipHash_Update(&ctx, &inc->inc_laddr, sizeof(inc->inc_laddr));
 1715                 break;
 1716 #endif
 1717 #ifdef INET6
 1718         case INC_ISIPV6:
 1719                 SipHash_Update(&ctx, &inc->inc6_faddr, sizeof(inc->inc6_faddr));
 1720                 SipHash_Update(&ctx, &inc->inc6_laddr, sizeof(inc->inc6_laddr));
 1721                 break;
 1722 #endif
 1723         }
 1724         SipHash_Update(&ctx, &inc->inc_fport, sizeof(inc->inc_fport));
 1725         SipHash_Update(&ctx, &inc->inc_lport, sizeof(inc->inc_lport));
 1726         SipHash_Update(&ctx, &flags, sizeof(flags));
 1727         SipHash_Update(&ctx, &secmod, sizeof(secmod));
 1728         SipHash_Final((u_int8_t *)&siphash, &ctx);
 1729 
 1730         return (siphash[0] ^ siphash[1]);
 1731 }
 1732 
 1733 static tcp_seq
 1734 syncookie_generate(struct syncache_head *sch, struct syncache *sc)
 1735 {
 1736         u_int i, mss, secbit, wscale;
 1737         uint32_t iss, hash;
 1738         uint8_t *secbits;
 1739         union syncookie cookie;
 1740 
 1741         SCH_LOCK_ASSERT(sch);
 1742 
 1743         cookie.cookie = 0;
 1744 
 1745         /* Map our computed MSS into the 3-bit index. */
 1746         mss = min(tcp_mssopt(&sc->sc_inc), max(sc->sc_peer_mss, V_tcp_minmss));
 1747         for (i = sizeof(tcp_sc_msstab) / sizeof(*tcp_sc_msstab) - 1;
 1748              tcp_sc_msstab[i] > mss && i > 0;
 1749              i--)
 1750                 ;
 1751         cookie.flags.mss_idx = i;
 1752 
 1753         /*
 1754          * Map the send window scale into the 3-bit index but only if
 1755          * the wscale option was received.
 1756          */
 1757         if (sc->sc_flags & SCF_WINSCALE) {
 1758                 wscale = sc->sc_requested_s_scale;
 1759                 for (i = sizeof(tcp_sc_wstab) / sizeof(*tcp_sc_wstab) - 1;
 1760                      tcp_sc_wstab[i] > wscale && i > 0;
 1761                      i--)
 1762                         ;
 1763                 cookie.flags.wscale_idx = i;
 1764         }
 1765 
 1766         /* Can we do SACK? */
 1767         if (sc->sc_flags & SCF_SACK)
 1768                 cookie.flags.sack_ok = 1;
 1769 
 1770         /* Which of the two secrets to use. */
 1771         secbit = sch->sch_sc->secret.oddeven & 0x1;
 1772         cookie.flags.odd_even = secbit;
 1773 
 1774         secbits = sch->sch_sc->secret.key[secbit];
 1775         hash = syncookie_mac(&sc->sc_inc, sc->sc_irs, cookie.cookie, secbits,
 1776             (uintptr_t)sch);
 1777 
 1778         /*
 1779          * Put the flags into the hash and XOR them to get better ISS number
 1780          * variance.  This doesn't enhance the cryptographic strength and is
 1781          * done to prevent the 8 cookie bits from showing up directly on the
 1782          * wire.
 1783          */
 1784         iss = hash & ~0xff;
 1785         iss |= cookie.cookie ^ (hash >> 24);
 1786 
 1787         /* Randomize the timestamp. */
 1788         if (sc->sc_flags & SCF_TIMESTAMP) {
 1789                 sc->sc_ts = arc4random();
 1790                 sc->sc_tsoff = sc->sc_ts - tcp_ts_getticks();
 1791         }
 1792 
 1793         TCPSTAT_INC(tcps_sc_sendcookie);
 1794         return (iss);
 1795 }
 1796 
 1797 static struct syncache *
 1798 syncookie_lookup(struct in_conninfo *inc, struct syncache_head *sch, 
 1799     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
 1800     struct socket *lso)
 1801 {
 1802         uint32_t hash;
 1803         uint8_t *secbits;
 1804         tcp_seq ack, seq;
 1805         int wnd, wscale = 0;
 1806         union syncookie cookie;
 1807 
 1808         SCH_LOCK_ASSERT(sch);
 1809 
 1810         /*
 1811          * Pull information out of SYN-ACK/ACK and revert sequence number
 1812          * advances.
 1813          */
 1814         ack = th->th_ack - 1;
 1815         seq = th->th_seq - 1;
 1816 
 1817         /*
 1818          * Unpack the flags containing enough information to restore the
 1819          * connection.
 1820          */
 1821         cookie.cookie = (ack & 0xff) ^ (ack >> 24);
 1822 
 1823         /* Which of the two secrets to use. */
 1824         secbits = sch->sch_sc->secret.key[cookie.flags.odd_even];
 1825 
 1826         hash = syncookie_mac(inc, seq, cookie.cookie, secbits, (uintptr_t)sch);
 1827 
 1828         /* The recomputed hash matches the ACK if this was a genuine cookie. */
 1829         if ((ack & ~0xff) != (hash & ~0xff))
 1830                 return (NULL);
 1831 
 1832         /* Fill in the syncache values. */
 1833         sc->sc_flags = 0;
 1834         bcopy(inc, &sc->sc_inc, sizeof(struct in_conninfo));
 1835         sc->sc_ipopts = NULL;
 1836         
 1837         sc->sc_irs = seq;
 1838         sc->sc_iss = ack;
 1839 
 1840         switch (inc->inc_flags & INC_ISIPV6) {
 1841 #ifdef INET
 1842         case 0:
 1843                 sc->sc_ip_ttl = sotoinpcb(lso)->inp_ip_ttl;
 1844                 sc->sc_ip_tos = sotoinpcb(lso)->inp_ip_tos;
 1845                 break;
 1846 #endif
 1847 #ifdef INET6
 1848         case INC_ISIPV6:
 1849                 if (sotoinpcb(lso)->inp_flags & IN6P_AUTOFLOWLABEL)
 1850                         sc->sc_flowlabel = sc->sc_iss & IPV6_FLOWLABEL_MASK;
 1851                 break;
 1852 #endif
 1853         }
 1854 
 1855         sc->sc_peer_mss = tcp_sc_msstab[cookie.flags.mss_idx];
 1856 
 1857         /* We can simply recompute receive window scale we sent earlier. */
 1858         while (wscale < TCP_MAX_WINSHIFT && (TCP_MAXWIN << wscale) < sb_max)
 1859                 wscale++;
 1860 
 1861         /* Only use wscale if it was enabled in the orignal SYN. */
 1862         if (cookie.flags.wscale_idx > 0) {
 1863                 sc->sc_requested_r_scale = wscale;
 1864                 sc->sc_requested_s_scale = tcp_sc_wstab[cookie.flags.wscale_idx];
 1865                 sc->sc_flags |= SCF_WINSCALE;
 1866         }
 1867 
 1868         wnd = sbspace(&lso->so_rcv);
 1869         wnd = imax(wnd, 0);
 1870         wnd = imin(wnd, TCP_MAXWIN);
 1871         sc->sc_wnd = wnd;
 1872 
 1873         if (cookie.flags.sack_ok)
 1874                 sc->sc_flags |= SCF_SACK;
 1875 
 1876         if (to->to_flags & TOF_TS) {
 1877                 sc->sc_flags |= SCF_TIMESTAMP;
 1878                 sc->sc_tsreflect = to->to_tsval;
 1879                 sc->sc_ts = to->to_tsecr;
 1880                 sc->sc_tsoff = to->to_tsecr - tcp_ts_getticks();
 1881         }
 1882 
 1883         if (to->to_flags & TOF_SIGNATURE)
 1884                 sc->sc_flags |= SCF_SIGNATURE;
 1885 
 1886         sc->sc_rxmits = 0;
 1887 
 1888         TCPSTAT_INC(tcps_sc_recvcookie);
 1889         return (sc);
 1890 }
 1891 
 1892 #ifdef INVARIANTS
 1893 static int
 1894 syncookie_cmp(struct in_conninfo *inc, struct syncache_head *sch,
 1895     struct syncache *sc, struct tcphdr *th, struct tcpopt *to,
 1896     struct socket *lso)
 1897 {
 1898         struct syncache scs, *scx;
 1899         char *s;
 1900 
 1901         bzero(&scs, sizeof(scs));
 1902         scx = syncookie_lookup(inc, sch, &scs, th, to, lso);
 1903 
 1904         if ((s = tcp_log_addrs(inc, th, NULL, NULL)) == NULL)
 1905                 return (0);
 1906 
 1907         if (scx != NULL) {
 1908                 if (sc->sc_peer_mss != scx->sc_peer_mss)
 1909                         log(LOG_DEBUG, "%s; %s: mss different %i vs %i\n",
 1910                             s, __func__, sc->sc_peer_mss, scx->sc_peer_mss);
 1911 
 1912                 if (sc->sc_requested_r_scale != scx->sc_requested_r_scale)
 1913                         log(LOG_DEBUG, "%s; %s: rwscale different %i vs %i\n",
 1914                             s, __func__, sc->sc_requested_r_scale,
 1915                             scx->sc_requested_r_scale);
 1916 
 1917                 if (sc->sc_requested_s_scale != scx->sc_requested_s_scale)
 1918                         log(LOG_DEBUG, "%s; %s: swscale different %i vs %i\n",
 1919                             s, __func__, sc->sc_requested_s_scale,
 1920                             scx->sc_requested_s_scale);
 1921 
 1922                 if ((sc->sc_flags & SCF_SACK) != (scx->sc_flags & SCF_SACK))
 1923                         log(LOG_DEBUG, "%s; %s: SACK different\n", s, __func__);
 1924         }
 1925 
 1926         if (s != NULL)
 1927                 free(s, M_TCPLOG);
 1928         return (0);
 1929 }
 1930 #endif /* INVARIANTS */
 1931 
 1932 static void
 1933 syncookie_reseed(void *arg)
 1934 {
 1935         struct tcp_syncache *sc = arg;
 1936         uint8_t *secbits;
 1937         int secbit;
 1938 
 1939         /*
 1940          * Reseeding the secret doesn't have to be protected by a lock.
 1941          * It only must be ensured that the new random values are visible
 1942          * to all CPUs in a SMP environment.  The atomic with release
 1943          * semantics ensures that.
 1944          */
 1945         secbit = (sc->secret.oddeven & 0x1) ? 0 : 1;
 1946         secbits = sc->secret.key[secbit];
 1947         arc4rand(secbits, SYNCOOKIE_SECRET_SIZE, 0);
 1948         atomic_add_rel_int(&sc->secret.oddeven, 1);
 1949 
 1950         /* Reschedule ourself. */
 1951         callout_schedule(&sc->secret.reseed, SYNCOOKIE_LIFETIME * hz);
 1952 }
 1953 
 1954 /*
 1955  * Returns the current number of syncache entries.  This number
 1956  * will probably change before you get around to calling 
 1957  * syncache_pcblist.
 1958  */
 1959 int
 1960 syncache_pcbcount(void)
 1961 {
 1962         struct syncache_head *sch;
 1963         int count, i;
 1964 
 1965         for (count = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
 1966                 /* No need to lock for a read. */
 1967                 sch = &V_tcp_syncache.hashbase[i];
 1968                 count += sch->sch_length;
 1969         }
 1970         return count;
 1971 }
 1972 
 1973 /*
 1974  * Exports the syncache entries to userland so that netstat can display
 1975  * them alongside the other sockets.  This function is intended to be
 1976  * called only from tcp_pcblist.
 1977  *
 1978  * Due to concurrency on an active system, the number of pcbs exported
 1979  * may have no relation to max_pcbs.  max_pcbs merely indicates the
 1980  * amount of space the caller allocated for this function to use.
 1981  */
 1982 int
 1983 syncache_pcblist(struct sysctl_req *req, int max_pcbs, int *pcbs_exported)
 1984 {
 1985         struct xtcpcb xt;
 1986         struct syncache *sc;
 1987         struct syncache_head *sch;
 1988         int count, error, i;
 1989 
 1990         for (count = 0, error = 0, i = 0; i < V_tcp_syncache.hashsize; i++) {
 1991                 sch = &V_tcp_syncache.hashbase[i];
 1992                 SCH_LOCK(sch);
 1993                 TAILQ_FOREACH(sc, &sch->sch_bucket, sc_hash) {
 1994                         if (count >= max_pcbs) {
 1995                                 SCH_UNLOCK(sch);
 1996                                 goto exit;
 1997                         }
 1998                         if (cr_cansee(req->td->td_ucred, sc->sc_cred) != 0)
 1999                                 continue;
 2000                         bzero(&xt, sizeof(xt));
 2001                         xt.xt_len = sizeof(xt);
 2002                         if (sc->sc_inc.inc_flags & INC_ISIPV6)
 2003                                 xt.xt_inp.inp_vflag = INP_IPV6;
 2004                         else
 2005                                 xt.xt_inp.inp_vflag = INP_IPV4;
 2006                         bcopy(&sc->sc_inc, &xt.xt_inp.inp_inc, sizeof (struct in_conninfo));
 2007                         xt.xt_tp.t_inpcb = &xt.xt_inp;
 2008                         xt.xt_tp.t_state = TCPS_SYN_RECEIVED;
 2009                         xt.xt_socket.xso_protocol = IPPROTO_TCP;
 2010                         xt.xt_socket.xso_len = sizeof (struct xsocket);
 2011                         xt.xt_socket.so_type = SOCK_STREAM;
 2012                         xt.xt_socket.so_state = SS_ISCONNECTING;
 2013                         error = SYSCTL_OUT(req, &xt, sizeof xt);
 2014                         if (error) {
 2015                                 SCH_UNLOCK(sch);
 2016                                 goto exit;
 2017                         }
 2018                         count++;
 2019                 }
 2020                 SCH_UNLOCK(sch);
 2021         }
 2022 exit:
 2023         *pcbs_exported = count;
 2024         return error;
 2025 }

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